1. Field of the Invention
The present invention pertains to improvements in high efficacy fill materials within bulbs of discharge lamps and has particular, although not limited, utility in lamps of the type disclosed in U.S. Pat. Nos. 5,404,076 and 5,606,220 (Dolan et al.) and PCT Publication WO 92/08240, the disclosures of which are incorporated herein by reference, in their entireties.
2. Related Art
Electrodeless lamps of the type with which the present invention is concerned are comprised of a light transmissive envelope containing a plasma-forming medium or fill. A microwave or radio frequency (RF) energy source has its output energy coupled to the envelope via a coupling waveguide to excite a plasma, resulting in a light discharge. In order to initiate breakdown (i.e., ignite the discharge), various techniques have been suggested. For example, in U.S. Pat. No. 4,359,668 (Ury), a supplemental ultraviolet igniter bulb, energized by extracting a portion of the primary microwave energy, emits energetic photons incident on the electrodeless lamp envelope. These photons ionize the fill within the envelope to effect the desired discharge therein.
Although ignition devices disposed outside the envelope can be effective, they consume space and add to the cost of the overall lamp. Even if an external or supplemental light source is used for starting, starting is not always reliable. It is desirable, therefore, to incorporate an additive in the fill material that has a low ionization potential and, therefore, facilitates initial breakdown so that the primary fill material can be ignited.
Other prior art systems used to assist lamp starting employ an arc discharge to free electrons through use of a conductive component to either concentrate a high strength electric field or introduce a concentrated field from an external power source. However, a system providing concentration or introduction of a high strength electric field requires additional components, thereby increasing cost.
Another approach involves the use of a fill additive which is partially electrically conductive at room temperature but non-conductive or a vapor at lamp operating temperatures. Such an approach is disclosed in U.S. Pat. No. 5,670,842 (Dolan, et al.).
FIG. 1 illustrates a typical configuration of an electrodeless lamp of the type with which the present invention is concerned. Specifically, a source or generator 16 generates microwave or RF energy and delivers the energy into a waveguide 18. The waveguide 18 directs the generated energy waves and couples the energy waves into a cavity 6, typically provided with a conductive mesh grid 8 for retaining the generated waves within the cavity 6 while allowing light waves to emanate therefrom. A transparent quartz bulb 10 in the cavity 6 is typically spherical or otherwise suitably configured and encloses a fill material containing sulfur and/or selenium, a trace amount of krypton-85 (e.g., 0.1 microcurie) and xenon gas and provides light when excited to form a plasma by the generated energy waves. The sulfur, selenium or sulfur/selenium mixture may be a solid having a low vapor pressure at room temperature and become gaseous with a high vapor pressure (e.g., two to twenty atmospheres) at typical lamp operating temperatures.
In operation, the radiation of the energy waves excites the fill atoms in the bulb 10 to effect a discharge of electrons. The discharged electrons collide with other fill atoms causing a further discharge of electrons, thereby increasing the total population of free electrons. The increased population of free electrons results in increased collisions and the process avalanches into radiation of light from a plasma. In the illustrated embodiment, the bulb 10 is connected by its elongate stem 12 to a motor 14 for rotating the bulb 10 about the longitudinal axis of the stem 12. In other embodiments, no mechanism for bulb rotation is required.
It is often necessary to restart or restrike the plasma ignition in a bulb as soon as possible after light discharge has been extinguished. Bulbs with fills of sulfur, selenium, a trace amount of krypton-85 and any quantity of xenon gas are difficult to start and difficult to restrike.